Mechanical processing such as cutting has been suitably employed for forming patterns of precision parts such as electronic parts, optical parts, and biochips. However, mechanical processing cannot keep pace with a recent trend towards high-resolution micropatterning and fails to attain a required precision. Thus, demand has arisen for a microprocessing technique that would replace mechanical processing. Meanwhile, in the field of semiconductor microprocessing, a microprocessing technique making use of a photoresist is employed as a high-precision processing method, and in recent years the technique has attained a very high precision on a sub-micron level. In another known high-aspect processing method, PMMA (polymethyl methacrylate) serves as a photoresist under irradiation with X-rays.
Although photoresists employed for microprocessing of a semiconductor are suitable for thin film processing, these resists are not suited for processing a thick film having a thickness of 50 μm or more, particularly 100 μm or more, required for producing a variety of parts. In addition, since these resists exhibit poor mechanical strength and poor weather resistance, resist patterns obtained from the resists cannot be used as permanent patterns. If a resist pattern could be used as a permanent pattern, the resist pattern itself can serve as a precision part without further processing. As a result, the number of steps required for producing precision parts would be greatly reduced, thereby providing industrial utility.
In contrast to PMMA, epoxy resins are suited for forming permanent patterns. Namely, epoxy resins have excellent mechanical strength, adhesion to a substrate, and weather resistance, and are employed as coating materials and materials for producing parts. One proposed method for processing epoxy resin is “photo-fabrication of three-dimensional objects” (see Japanese Patent Application Laid-Open (kokai) Nos. 60-247515 and 5-24119). Materials which are applicable to such a method are disclosed in, for example, Japanese Patent Application Laid-Open (kokai) No. 2000-239309.
The photo-fabrication of three-dimensional objects includes repeated steps for selectively irradiating liquid photocurable resin with a laser beam or a similar beam so as to form a cured resin layer, to thereby form three-dimensional objects. However, the method, which is a type of direct writing method, is not suited for large-scale production.
Another known method for processing epoxy resin is a production method for printed circuit boards disclosed in Japanese Patent Publication (kokoku) No. 7-78628. The photoresist used in the production is a commercial product, SU-8 (trade name), which is characterized in that the product can form a pattern of a thicker film as compared with other known photoresists.
However, SU-8 (trade name) has a notable drawback. Specifically, SU-8 contains a novolak epoxy resin and a radiation-sensitive cationic initiator as predominant ingredients, and the novolak epoxy resin is readily contaminated with Cl and Na during its production process. Because of such contamination, the formed resist pattern has poor electrical characteristics and therefore is not suited for a permanent pattern. SU-8 also has other problems, in that it exhibits intense absorption in a deep UV region (wavelength: ≦300 nm) attributed to an aromatic ring included in the skeleton of novolak epoxy resin used in the material and that it has insufficient transparency in the UV region (wavelength: ≧300 nm) and the visible light region. Therefore, production of optical parts and biochips from the photoresist is difficult, and limitations are imposed on the wavelength of exposure light, which is also problematic.
Studies conducted in recent years have confirmed that, in semiconductor microprocessing, shifting the wavelength of exposure light to a shorter wavelength in the UV region effectively enhances pattern precision. Therefore, another demerit of SU-8 is that it cannot be used in the deep UV region. Furthermore, a cationic initiator must be selected in accordance with light absorption (transparency) of the resin. Since most commercial cationic initiators have absorption bands similar in wavelength range to those of novolak epoxy resin, the initiator must be selected from a limited range of commercial cationic initiators.
Some commercially available monomer products for producing an aliphatic epoxy resin have no aromatic group and contain only trivial amounts of migrated Cl and Na. Examples include glycidyl (meth)acrylate, CYCLOMER A200, and CYCLOMER M100 ((meth)acrylate having an aliphatic epoxy group, products of Daicel Chemical Industries, Ltd.), and Celloxide 2000 (1-vinyl-3,4-epoxycyclohexane, product of Daicel Chemical Industries, Ltd.). These monomers are polymerized through radical polymerization or a similar method, to thereby synthesize epoxy resins.
However, (meth)acrylates such as glycidyl (meth)acrylate, CYCLOMER A200, and CYCLOMER M100 have a (meth)acrylate ester backbone and are known to have relatively high sensitivity to high-energy active beams such as electron beams, deep UV rays, and X-rays. When the (meth)acrylates are irradiated with any such active beams, a side reaction other than the target epoxy-group-polymerization occurs in the backbone, greatly varying and affecting physical properties (e.g., patterning characteristics, sensitivity to exposure, characteristics of cured products) of the produced resins. Thus, such a high sensitivity is not preferred. Celloxide 2000 has no (meth)acrylate backbone, but raises concerns over its toxicity. Therefore, it must be used under strict control.
The processing of PMMA through an X-ray for producing a high-aspect pattern has also drawbacks; i.e., use of an X-ray based on a particular light source, and a very long processing time stemming from the low photoresist sensitivity of PMMA.